Microfluidic analytical systems have been a subject of increasing interest in recent years, particularly for the analysis of biomolecules. Devices have been reported using high performance liquid chromatography, electrophoreses, isoelectric focusing, and electrochromatography separations with photometric, fluorometric, electrochemical, and mass spectrometric (MS) detection methods. Applications of MS detection have been focused upon electrospray ionization (ESI), and several groups have reported the development of microfluidic systems for interfacing to ESI-MS.
Some of the earliest work described electrospray directly from microfluidic channels opening at the edge of a glass device. This approach was complicated by the tendency to accumulate a droplet at the exit that formed a mixing volume and degraded the resolution of the separation system on the device. Most of microfluidic devices interfaced to ESI-MS, however, have utilized conventional electrospray emitters (e.g. tapered fused silica capillaries) attached to the device. This approach yields satisfactory electrospray ionization performance, but has two problems: (i) the potential for dead volume in the attachment leading to degradation of separation quality, and (ii) the loss of the key advantage of photolithography-based microfabrication methods, i.e. the ability to make multiples of a function as easily as producing a single function on a device.
A few research groups have reported microfabricated electrospray ionization sources as an integral part of the device. One research group developed silicon nitride and parylene electrospray emitters microfabricated on silicon devices. An electrospray ionization emitter for an isoelectric focusing device has been constructed on polycarbonate plates using laser micromachining method. Recently, electrospray nozzles have been fabricated from a monolithic silicon substrate. These microfabricated electrospray ionization devices gave good electrospray performance, but all of them require relatively complex processes and facilities to produce the devices.
An object of the invention is to produce a microfluidic device for ESI-MS analysis, such as the analysis of peptides and proteins. Another object is to produce inexpensive, disposable devices for high throughput proteomics work. The electrospray emitter should be resistant to clogging, enhance durability and reliability, and extend the emitter's useful life and its range of applicability. The electrospray ionization efficiency should be at least comparable to conventional nano emitters, and should be useful in interfacing micro column liquid chromatography to mass spectrometry.